Manila: The United States is scaling down a taskforce that has been training Philippine troops to hunt down local Al-Qaeda extremists, Manila`s defence chief said Thursday.

The group of 500-600 American servicemen rotating through the strife-torn southern Philippines was cut back to 200 starting this year, Defence Secretary Voltaire Gazmin told reporters.

"They were never permanent. They just teach, they train our people and they are satisfied with what we have learned so they are reducing their numbers," he said.

Gazmin said the work of Filipino troops had improved sharply since their US counterparts, primarily special forces, started rotating through the south around 2002, adding that the Americans had also learned "jungle warfare" from the Filipinos.

Gazmin also said that American training and loans of US equipment had played a crucial role in weakening the Abu Sayyaf, a Filipino Muslim extremist group linked to the worst terror attacks in the country`s history.

This includes the bombing of a ferry in Manila in 2004 that killed more than 100 people, as well as dozens of kidnappings in the remote and Muslim-populated south, often targeting foreigners including Americans.

Founded in the 1990s with seed money from Osama bin Laden`s Al-Qaeda network, the Abu Sayyaf has survived over the past decade by drawing support from poor Muslim communities that have become a fertile recruiting ground.

But it has suffered serious setbacks in recent weeks, including the arrest of one of its top leaders Khair Mundos, listed by the US government as one of its "most wanted" terrorists with a $500,000 reward on his head.

The cutback of US troops in the southern Philippines comes after Washington and Manila signed a new defence agreement in April, allowing American forces greater access to bases in its former colony.

Washington announced a "pivot" towards Asia in 2011, including a stronger military presence.

The defence pact also comes as the Philippines seeks more US help in dealing with territorial disputes with an increasingly assertive China.